Using the time intelligence functions in Power BI is a must to do any type of analysis in Power BI. One of the mosy useful is the DATEADD function. When coupled with the CALCULATE function it's one of the more flexible formulas in Power BI. This tutorial is should you how to create a KeyMeasures table then create a Measures to work out the previous years sales. Then you'll create aother measure to calculate the difference in sales between this years and last years sales. Finally you will create a measure that will convert that value into a percentage.
Should you want to follow along with the tutorial then you can download the Power BI - previous year exercise file.